A wind turbine known in the art comprises a tapered wind turbine tower and a wind turbine nacelle positioned on top of the tower. A wind turbine rotor with a number of wind turbine blades is connected to the nacelle through a low speed shaft, which extends out of the nacelle front as illustrated on FIG. 1.
Oscillations and vibrations of the wind turbine blades are undesirable in that in worst-case they can damage the blades. In particular edge-wise oscillations, which are oscillations along the chord between the trailing edge and the leading edge of the blade, can damage the blade, in that the blade has little damping towards this mode of oscillations.
Furthermore, edgewise oscillations are particularly harmful, in that they among other things can cause cracks at the root of the blade or along the trailing edge. In known cases such oscillations has caused the blade to fail to such degree, that the blade has disintegrated from the turbine.
Both stall and pitch controlled wind turbine are in risk of being damaged by edge-wise oscillations. The stall controlled turbine is mostly seeing this problem when operating in high winds beyond the stall point and the pitch regulated turbine is mostly seeing this problem in high wind where sudden wind gusts can cause the blades to stall momentarily.
To eliminate harmful oscillations of the blades it is known to shut down the wind turbine for a period of time, if potentially damaging edgewise oscillations of the blades is detected. But if these oscillations are detected often, this method will seriously reduce the overall output of the wind turbine.
It is also known to provide the blades with different forms of mechanical dampers, most often based on the principle of a spring mounted mass combined with a damping device or they can be provided with different kinds of liquid dampers.
An example of a liquid damper is disclosed in WO 99/32789, where the tips of the blades are provided with a tuned liquid damper system. A liquid flows freely in a number of cambers placed as close to the tip of the blade as possible. The chambers have a specific length, which is adapted to the natural edgewise frequency of the specific blade type. Even though these kinds of frequency specific dampers weigh less than traditional multi-frequency dampers, they still have the disadvantage of adding considerable weight to the tip of the blade, where weight is least desired and under all circumstances it is undesired to provide anything that can break down in the blades, both because the inside of the blades can be very difficult to access and because any extra weight in the blades is undesired.
An object of the invention is to provide for a wind turbine comprising means for damping or eliminating edgewise oscillations in the blades, which do not present the mentioned disadvantages.
Furthermore, it is an object of the invention to provide for a simple and cost-efficient technique for damping or eliminating edgewise oscillations of one or more blades of a wind turbine.